Semiconductor photodetectors that use the “avalanche effect” for signal amplification have areas of high electrical field strength in a near-surface region of the semiconductor substrate, and these areas help to multiply charge carriers that are generated by radiation absorption in the semiconductor substrate. The areas of high electrical field strength are created for example by forming doping zones that have been doped according to different doping types and are assigned to each other within the semiconductor substrate of the photodetector.
In order to detect extremely small quantities of radiation, down to the level of single photons, such semiconductor photodetectors are operated with a bias voltage higher than the voltage that causes permanent breakdown of the component structures. When the semiconductor photodetector is operated, after a certain time thermally generated charge carriers or charge carriers generated by radiation absorption penetrate the area of high electrical field strength and are multiplied there by “avalanche breakdown”, which causes a high current between the electrical connectors or contacts of the semiconductor photodetector. If the voltage at the electrical contacts of the photodetector is not lowered and if internal serial resistances within the semiconductor photodetector do not bring about a reduction in the high field strength, the breakdown becomes permanent, since new charge carriers are created constantly in the resulting charge carrier avalanche.
However, if a serial resistance is interposed between the operating voltage and the contacts of the semiconductor photodetector, the field strength in the area of high electrical field strength may be reduced by the current pulse and the associated voltage drop in such manner that permanent avalanche multiplication can no longer be sustained. Consequently, the current falls and the high field strength in the area of high field strength is established again. Such a serial resistance is also referred to as a quench resistance.
All of the processes described are time-dependent. For semiconductor photodetectors with a relatively large detector array, switching times or recovery times between the triggering of a charge carrier avalanche and quenching of the avalanche, that is to say the time before a single incident can be registered again, is very long. It was therefore suggested to divide the active surface of the semiconductor photodetector in a large number of individual pixel elements and to assign a quench resistance to each pixel element (see for example Sadygov Z.: “Three advanced designs of micro-pixel avalanche photodiodes: Their present status, maximum possibilities and limitations”, Nuclear Instruments and Methods in Physics Research A 567 (2006)70-73). In a structural variant of known avalanche photodiodes, the quench resistance is partially in the area of the radiation penetration window. This causes disadvantages with regard to the usable detector area, since this is limited by resistance layers and metal contacts.
It was suggested in document DE 10 2007 037 020 B3 to form the quench resistance in the semiconductor substrate of the photodetector, between the area of high field strength and a contact layer on the back side. The quench resistance is thus located deep inside the semiconductor substrate. However, this construction has the disadvantage that highly specific requirements are imposed on the design of the semiconductor substrate, and particular dependence on material parameters and structure sizes of the pixel elements arises.
Document DE 10 2007 037 020 B3 discloses an avalanche photodiode for detecting radiation.
A single photon avalanche photodiode is described in the document WO 2008/011617.